U.S. patent number 10,658,146 [Application Number 16/536,059] was granted by the patent office on 2020-05-19 for transmission type target, transmission type target unit, xray tube, x-ray generating apparatus, and radiography system.
This patent grant is currently assigned to Canon Kabushiki Kaisha. The grantee listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Yoichi Ikarashi, Takao Ogura, Tatsuya Suzuki, Takeo Tsukamoto, Shuji Yamada, Tadayuki Yoshitake.
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United States Patent |
10,658,146 |
Yamada , et al. |
May 19, 2020 |
Transmission type target, transmission type target unit, xray tube,
X-ray generating apparatus, and radiography system
Abstract
A radiation emitting target, a radiation generating device, and
a radiography system are provided in which adhesion between a
target layer and a diamond substrate is improved and stable
radiation emitting properties are exhibited. A transmission type
target includes a target layer, a carbon-containing region
including sp2 bonds, and a diamond substrate that supports the
target layer. The carbon-containing region is positioned between
the target layer and the diamond substrate.
Inventors: |
Yamada; Shuji (Abiko,
JP), Suzuki; Tatsuya (Kawasaki, JP),
Tsukamoto; Takeo (Atsugi, JP), Ikarashi; Yoichi
(Fujisawa, JP), Yoshitake; Tadayuki (Tokyo,
JP), Ogura; Takao (Yokohama, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
N/A |
JP |
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Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
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Family
ID: |
50179498 |
Appl.
No.: |
16/536,059 |
Filed: |
August 8, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190385808 A1 |
Dec 19, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14204810 |
Mar 11, 2014 |
10418222 |
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Foreign Application Priority Data
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Mar 12, 2013 [JP] |
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2013-049350 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
35/08 (20130101); H01J 35/18 (20130101); H01J
2235/084 (20130101); H05G 1/06 (20130101); H01J
35/116 (20190501) |
Current International
Class: |
H01J
35/18 (20060101); H01J 35/08 (20060101); H05G
1/06 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kao; Chih-Cheng
Attorney, Agent or Firm: Canon U.S.A., Inc., IP Division
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent Ser. No.
14/204,810, filed Mar. 11, 2014, which claims the benefit of
Japanese Patent Application No. 2013-049350, filed Mar. 12, 2013
each of which is hereby incorporated by reference herein in its
entirety.
Claims
What is claimed is:
1. A transmission type target for an X-ray tube comprising: a
diamond substrate including an inner region which contains sp3
bonds and an outer region which contains a higher amount of sp2
bonds than the inner region; and a target layer located on the
diamond substrate and configured to generate an X-ray in response
to irradiation with electrons.
2. The transmission type target according to claim 1, wherein the
outer region comprises a carbon compound comprising sp2 bonds in a
cyclic, linear, or three dimensional network main chain, or an
allotrope of diamond, the allotrope comprising sp2 bonds.
3. The transmission type target according to claim 1, wherein the
outer region comprises at least one selected from amorphous carbon,
glassy carbon, diamond-like carbon, graphene, graphite, carbon
nanotubes, graphite nanofibers, and fullerene.
4. The transmission type target according to claim 1, wherein the
outer region forms a part of the diamond substrate.
5. The transmission type target according to claim 1, wherein the
outer region is a continuous layer positioned.
6. The transmission type target according to claim 1, wherein the
outer region has a thickness 0.005 times to 0.1 times a thickness
of the target layer.
7. The transmission type target according to claim 1, wherein the
target layer contains at least one metal element selected from the
group consisting of tantalum, tungsten, and molybdenum.
8. A transmission type target unit comprising: the transmission
type target according to claim 1; and an anode member connected to
a periphery of the diamond substrate and electrically connected to
the target layer.
9. An X-ray tube comprising: the transmission type target unit
according to claim 8; an electron emitting source comprising an
electron emitting portion disposed to face the target layer; and an
envelope that houses the electron emitting portion and the target
layer in an interior space of the envelope or on an inner surface
of the envelope.
10. An X-ray generating apparatus comprising: the X-ray tube
according to claim 9, and a drive circuit electrically connected to
the target layer and the electron emitting portion and configured
to output a tube voltage to be applied between the target layer and
the electron emitting portion.
11. A radiography system comprising: the radiation generating
apparatus according to claim 10; and a radiation detector
configured to detect radiation that has been released from the
radiation generating apparatus and has passed through a
specimen.
12. A transmission type target for an X-ray tube comprising: a
target layer configured to generate an X-ray in response to
irradiation with electrons; and a diamond substrate including an
interior region which contains sp3 bonds, a supporting surface
configured to support the target layer and a stress relaxation
region between the interior region and the supporting surface in a
thickness direction of the target layer, the stress relaxation
region containing a higher amount of sp2 bonds than the interior
region.
13. The transmission type target according to claim 12, wherein the
stress relaxation region comprises a carbon compound comprising sp2
bonds in a cyclic, linear, or three dimensional network main chain,
or an allotrope of diamond, the allotrope comprising sp2 bonds.
14. The transmission type target according to claim 12, wherein the
stress relaxation region comprises at least one selected from
amorphous carbon, glassy carbon, diamond-like carbon, graphene,
graphite, carbon nanotubes, graphite nanofibers, and fullerene.
15. The transmission type target according to claim 12, wherein the
stress relaxation region forms a part of the diamond substrate.
16. The transmission type target according to claim 12, wherein the
stress relaxation region is a continuous layer.
17. The transmission type target according to claim 12, wherein the
stress relaxation region has a thickness 0.005 times to 0.1 times a
thickness of the target layer.
18. The transmission type target according to claim 12, wherein the
target layer contains at least one metal element selected from the
group consisting of tantalum, tungsten, and molybdenum.
19. A transmission type target unit comprising: the transmission
type target according to claim 12; and an anode member connected to
a periphery of the diamond substrate and electrically connected to
the target layer.
20. An X-ray tube comprising: the transmission type target unit
according to claim 19; an electron emitting source comprising an
electron emitting portion disposed to face the target layer; and an
envelope that houses the electron emitting portion and the target
layer in an interior space of the envelope or on an inner surface
of the envelope.
21. An X-ray generating apparatus comprising: the X-ray tube
according to claim 20, and a drive circuit electrically connected
to the target layer and the electron emitting portion and
configured to output a tube voltage to be applied between the
target layer and the electron emitting portion.
22. A radiography system comprising: the radiation generating
apparatus according to claim 21; and a radiation detector
configured to detect radiation that has been released from the
radiation generating apparatus and has passed through a specimen.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to radiation generating apparatus
that are used in medical equipment for diagnosis applications and
industrial equipment for non-destructive X-ray imaging, for
example.
The present invention particularly relates to a transmission X-ray
target that includes a target layer and a diamond substrate
supporting the target layer, a radiation generating tube that
includes the transmission X-ray target, a radiation generating
apparatus that includes the radiation generating tube, and a
radiography system that includes the radiation generating
apparatus.
Description of the Related Art
Radiation generating apparatus that generate X-rays used in medical
diagnosis desirably have enhanced durability and require less
maintenance so as to raise the generators' operating rate and serve
as medical modality that can be used in home medical care and
emergency medicine in the event of disasters and accidents.
One of the main factors that determine the durability of radiation
generating apparatus is a heat-resistance of targets used as
sources for generating radiation.
In a radiation generating apparatus configured to generate
radiation by irradiating a target with an electron beam, the
radiation generation efficiency at the target is less than 1% and
almost all of the energy applied to the target is converted to
heat. If heat generated at the target is not sufficiently released,
adhesion of the target may decrease due to thermal stress and the
heat-resistance of the target becomes limited.
One known way of improving the radiation generation efficiency of a
target is to use a transmission type target that includes a target
layer, which is in a form of a thin film that contains a heavy
metal, and a substrate that transmits the radiation and supports
the target layer. PCT Japanese Translation Patent Publication No.
2009-545840 discloses a rotating anode-type transmission type
target offering a radiation generation efficiency at least 1.5
times higher than that of known rotating anode-type reflection
targets.
One known way of accelerating the release of heat from the target
to outside is to use diamond as a substrate that supports the
target layer of a multilayered target. PCT Japanese Translation
Patent Publication No. 2003-505845 discloses use of diamond as the
substrate that supports a target layer composed of tungsten, by
which the heat releasing property is enhanced and microfocusing is
realized. Diamond has not only high heat-resistance and high heat
conductivity but also a high radiation transmitting property, and
is thus suitable as a material for the substrate that supports a
transmission type target on one hand.
On the other hand, diamond has low wettability to molten metal and
linear expansion coefficient mismatch occurs between diamond and
solid metal. Thus, diamond has low affinity to a target metal.
Ensuring the adhesion between a target layer and a diamond
substrate has been the issue that needs to be addressed in
improving the reliability of transmission type targets.
PCT Japanese Translation Patent Publication No. 2003-505845
discloses a transmission type target in which an intermediate layer
whose material is not disclosed is provided as an adhesion
promoting layer and interposed between a diamond substrate and a
target layer.
Japanese Patent Laid-Open No. 2002-298772 discloses that thermal
stress occurs between a target layer and a diamond substrate due to
linear expansion coefficient mismatch in a radiation generating
tube equipped with a transmission type target and that separation
and cracks occur in the target layer due to the thermal stress.
Japanese Patent Laid-Open No. 2002-298772 discloses a structure in
which a target layer is warped toward a diamond substrate so that
during operation of a radiation generating tube, the target layer
is pressed toward the diamond substrate and separation of the
target layer is suppressed thereby.
Even the transmission type targets in which adhesion between a
target layer and a diamond substrate is enhanced and which are
disclosed in PCT Japanese Translation Patent Publication No.
2003-505845 and Japanese Patent Laid-Open No. 2002-298772 sometimes
suffer microcracks in the target layer, resulting in fluctuation in
radiation output.
FIGS. 10A and 10B are respectively a plan view and a
cross-sectional view of a transmission type target unit 71 that has
come to cause fluctuations in radiation output. FIGS. 10A and 10B
are obtained by microscopic observation and are provided as a
reference example. The transmission type target unit 71 shown in
FIGS. 10A and 10B is removed from a radiation generating apparatus
not shown in the drawing after a cumulative total of 103 times of
exposure. FIG. 10B is a cross-sectional view of the transmission
type target unit 71 taken along line XB-XB in the plan view of FIG.
10A.
In the transmission type target unit 71 of the reference example, a
microcrack 68 has branches extending at random within a region that
corresponds to the irradiation spot of an electron beam not shown
in the drawing, and forms a damaged region 67.
More detailed observation of the microcrack 68 within the damaged
region 67 revealed that, as shown in FIG. 10B, the microcrack 68
propagates from the upper surface to the lower surface of a target
layer 62. As shown in FIG. 10A, in a plane parallel to a target
layer 42, a microcrack 68 that has closed loops and island regions
65 and 65' divided by the closed loops of the microcrack 68 are
observed. The island regions 65 and 65' are regions where
electrical connection to an anode member 49 is not established.
No microcracks 68 were observed in the target layer 62 before the
transmission type target unit 71 is assembled into a radiation
generating apparatus not shown in the drawings. In the initial
stage after the transmission type target unit 71 was assembled into
the radiation generating apparatus, no fluctuation in radiation
output was observed. Accordingly, generation of the microcrack 68
observed in the target layer 62 and fluctuation in radiation output
are presumably caused by driving of the radiation generating
apparatus.
In this specification, a "microcrack" refers to a crack that
disrupts the continuity of a target layer when observed with a
microscope. Such a crack is observed as a local region where higher
light scattering is observed if an optical microscope is used, or
as a difference in contrast indicating presence of microscopic and
discrete voids if a scanning electron microscope such as a scanning
electron microscope (SEM), a scanning transmission electron
microscope (STEM), or a scanning transmission microscope (STM) is
used.
In the observation results shown in FIGS. 10A and 10B, an
interlayer is not provided between the target layer 62 and a
diamond substrate 61. However, in samples in which a titanium
interlayer was formed between the target layer 62 and the diamond
substrate 61 also, a similar microcrack 68 and a damaged region 67
that included an island region 65 were sometimes observed.
As described above, the inventors of the present invention have
found that as the radiation generating apparatus is driven,
microcracks occur in a target layer, performance of the target in
maintaining the anode potential at the target layer is degraded,
the tube current (anode current) flowing in the target layer 62
becomes unstable, and radiation output from the target layer 62
fluctuates as the driving history of the radiation generating
apparatus accumulates.
SUMMARY OF THE INVENTION
The present invention provides a highly reliable transmission type
target that has advantages of a transmission type target including
a diamond substrate and that suffers less from microcracks in a
target layer resulting from operation of a radiation generating
tube.
The present invention also provides a highly reliable transmission
type target with which an anode potential at the target layer is
stabilized and radiation output fluctuations are suppressed. The
present invention also provides highly reliable radiation
generating tube, radiation generating apparatus, and radiography
system with which output fluctuations are suppressed.
An aspect of the invention provides a transmission type target that
includes a target layer which is configured to generate an X-ray
upon irradiation with electrons and contains a target metal, a
carbon-containing region including sp2 bonds, and a diamond
substrate configured to support the target layer. The
carbon-containing region is positioned between the target layer and
the diamond substrate.
Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference
to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic cross-sectional view of a basic structural
example of a transmission type target according to an embodiment
and FIG. 1B is a schematic cross-sectional view of the transmission
type target in an operating state.
FIG. 2A and FIG. 2B are schematic cross-sectional views of a
section specimen 55 in Example 1 and show the positional
relationship between analyzed regions 145 and 146; FIG. 2C shows a
STEM-EELS profile; and FIG. 2D is a calibration line data used in
obtaining a normalized sp2 bond concentration.
FIG. 3A is a schematic diagram that shows a radiation generating
tube equipped with a transmission type target according to an
embodiment; FIG. 3B is a schematic diagram that shows a radiation
generating apparatus; and FIG. 3C is a schematic diagram that shows
a radiography system.
FIGS. 4A to 4E show other structural examples of the transmission
type targets.
FIGS. 5A-1 to 5E-3 show examples of embodiments of methods for
producing a transmission type target.
FIGS. 6A-1 to 6D-4 show examples of embodiments of methods for
producing a transmission type target.
FIG. 7 is a schematic diagram of a system for evaluating stability
of radiation output from a radiation generating apparatus of each
Example.
FIGS. 8A to 8C are conceptual renderings of grain boundary energy
distribution and grain diameters in a target layer at the initial
stage (8A), the intermediate stage (8B), and the later stage
(8C).
FIGS. 9A and 9B are schematic cross-sectional views showing the
relationship between an electron penetration length and a target
layer thickness (FIG. 9A: transmission type target, FIG. 9B:
reflection target) and FIGS. 9C and 9D are schematic
cross-sectional views showing the relationship between a depth of a
microcrack and a target layer thickness (FIG. 9C: transmission type
target, FIG. 9D: reflection target).
FIGS. 10A and 10B are a plan view and a cross-sectional view,
respectively, of a transmission type target in which microcracks
occurred in a target layer.
DESCRIPTION OF THE EMBODIMENTS
Preferred embodiments of the present invention will now be
described in detail with reference to the attached drawings. The
dimensions, materials, shapes, relative positions, and other
attributes of the constitutional members described in these
embodiments do not limit the scope of the present invention.
FIG. 3A is a cross-sectional view of an example of a radiation
generating tube equipped with a transmission type target according
to an embodiment of the invention and FIG. 3B is a cross-sectional
view of an example of a radiation generating apparatus.
Radiation Generating Tube
FIG. 3A shows an embodiment of a transmission-type radiation
generating tube 102 that includes an electron emitting source 3 and
a transmission type target 9 (hereinafter a transmission type
target is simply referred to as a "target" in this specification)
that faces the electron emitting source 3 with a space
therebetween.
In this embodiment, an electron beam 5 emitted from an electron
emitting portion 2 of the electron emitting source 3 hits a target
layer 42 of the target 9 and a radiation flux 11 is generated as a
result.
Electrons in the electron beam 5 are accelerated by an acceleration
electric field between the electron emitting source 3 and the
target layer 42 up to incident energy needed to generate radiation.
The acceleration electric field is generated in an inner space 13
of the radiation generating tube 102 due to a drive circuit 103
that outputs a tube voltage Va and a cathode and an anode
electrically connected to the drive circuit 103. In other words,
the tube voltage Va output from the drive circuit 103 is applied
between the target layer 42 and the electron emitting portion
2.
In this embodiment, as shown in FIG. 3A, the target 9 is
constituted by the target layer 42 and a diamond substrate 41 that
supports the target layer 42. A target unit 51 at least includes
the target 9 and an anode member 49 and functions as an anode of
the radiation generating tube 102.
The details of the target 9 and the target unit 51 are described
later.
A vacuum atmosphere is created in the inner space 13 of the
radiation generating tube 102 in order to guarantee the mean free
path of the electron beam 5. The degree of vacuum in the radiation
generating tube 102 is preferably 10.sup.-8 Pa or more and
10.sup.-4 Pa or less and, from the viewpoint of the lifetime of the
electron emitting source 3, more preferably 10.sup.-8 Pa or more
and 10.sup.-6 Pa or less.
The interior of the radiation generating tube 102 can be evacuated
by using a vacuum pump not shown in the drawings via an exhaust
duct not shown in the drawing and then sealing the exhaust duct. A
getter not shown in the drawings may be installed inside the
radiation generating tube 102 to maintain the degree of vacuum.
The radiation generating tube 102 includes an insulation tube 110
that serves as a body that electrically insulates between the
electron emitting source 3 at a cathode potential and the target
layer 42 at an anode potential. The insulation tube 110 is composed
of an insulating material such as a glass material or a ceramic
material. In this embodiment, the insulation tube 110 defines the
distance between the electron emitting source 3 and the target
layer 42.
The radiation generating tube 102 may be constituted by an envelope
that is airtight and has an atmospheric pressure resisting strength
in order to maintain the degree of vacuum. In this embodiment, the
envelope is constituted by the insulation tube 110, a cathode
equipped with the electron emitting source 3, and an anode equipped
with the target unit 51. The electron emitting portion 2 and the
target layer 42 are disposed in the inner space 13 or on an inner
wall of the envelope.
In this embodiment, the diamond substrate 41 serves as a
transmission window for allowing radiation generated at the target
layer 42 to go out of the radiation generating tube 102 and also as
a part of the envelope.
The electron emitting source 3 is positioned to face the target
layer 42 of the target 9. A hot cathode such as a tungsten filament
or an impregnated type cathode or a cold cathode such as a carbon
nanotube may be used as the electron emitting source 3. The
electron emitting source 3 may be equipped with a grid electrode or
an electrostatic lens electrode (not shown in the drawings) so as
to control the beam diameter and electron current density of the
electron beam 5, the ON/OFF timing, and the like.
Radiation Generating Apparatus
FIG. 3B shows an embodiment of a radiation generating apparatus 101
that emits a radiation flux 11 from a radiation transmitting window
121. In the radiation generating apparatus 101 of this embodiment,
the radiation generating tube 102 that serves as a radiation source
and a drive circuit 103 configured to drive the radiation
generating tube 102 are housed in a container 120 that has the
radiation transmitting window 121.
A tube voltage Va is applied between the target layer 42 and the
electron emitting portion 2 from the drive circuit 103 illustrated
in FIG. 3B. An appropriate tube voltage Va may be selected in
accordance with the thickness of the target layer 42 and the metal
type of the target so as to form a radiation generating apparatus
101 that generates a desired type of line.
The container 120 housing the radiation generating tube 102 and the
drive circuit 103 desirably has a sufficient strength as a housing
and a good heat releasing property. For example, the container 120
may be composed of a metal material such as brass, iron, or
stainless steel.
In this embodiment, an insulating liquid 109 fills an extra space
43 inside the container 120 and surrounding the radiation
generating tube 102 and the drive circuit 103. The insulating
liquid 109 has an electrical insulating property, maintains the
electrical insulation inside the container 120, and serves as a
cooling medium for the radiation generating tube 102. An
electrically insulating oil such as a mineral oil, a silicone oil,
or a perfluoro-based oil may be used as the insulating liquid
109.
Radiography System
Next, an example of a radiography system 60 equipped with a target
according to an embodiment of the invention is described with
reference to FIG. 3C.
A system control unit 202 is configured to integrally control the
radiation generating apparatus 101 and a radiation detector 206.
The drive circuit 103 under control of the system control unit 202
outputs various control signals to the radiation generating tube
102. Although the drive circuit 103 is housed in the container 120
of the radiation generating apparatus 101 together with the
radiation generating tube 102 in this embodiment, the drive circuit
103 may alternately be disposed outside the container 120. In
response to the control signals output from the drive circuit 103,
the state of emission of the radiation flux 11 emitted from the
radiation generating apparatus 101 is controlled.
The radiation flux 11 emitted from the radiation generating
apparatus 101 has its irradiation range adjusted by a collimator
unit (not shown) equipped with a movable aperture, is emitted
outside the radiation generating apparatus 101, passes through a
specimen 204, and detected with the radiation detector 206. The
radiation detector 206 converts the detected radiation into an
image signal and outputs the image signal to a signal processor
205.
The signal processor 205 processes the image signal under control
of the system control unit 202 and outputs the processed image
signal to the system control unit 202.
Based on the processed image signal, the system control unit 202
outputs to a display device 203 a display signal for displaying an
image in the display device 203.
The display device 203 displays the image based on the display
signal so as to show a captured image of the test object 204.
A representative example of the radiation discussed in this
specification is an X-ray. The radiation generating apparatus 101
and the radiography system according to embodiments of the
invention can be used as an X-ray generating unit and an X-ray
imaging system. X-ray imaging systems can be used in
non-destructive inspection of industrial products and medical
diagnosis of human and animals.
Target
Next, a structure and operation state of a basic embodiment of a
target of the invention are described with reference to FIGS. 1A
and 1B.
According to an embodiment shown in FIG. 1A, a target 9 at least
includes a target layer 42 containing a target metal and a diamond
substrate 41 that supports the target layer 42. The diamond
substrate 41 has a region 46 constituted by sp3 bonds. The target
layer 42 has a diamond-substrate-41-side portion connected to a
carbon-containing region 45 having sp2 bonds. In this
specification, the target 9 illustrated in FIG. 1A is a first
embodiment of the invention.
FIG. 1B shows an operation state of the target 9 illustrated in
FIG. 1A. One of the surfaces of the target layer 42 is irradiated
with an electron beam 5 and thus radiation is emitted in a radial
manner. The target 9 is a transmission type target with which some
of components are selected with a collimator or the like (not shown
in the drawing) from the components of the radiation emitted from
the target layer 42 and passed through the diamond substrate 41 in
the substrate thickness direction, and output as the radiation flux
11.
Either natural diamond or a synthetic diamond prepared by chemical
vapor deposition (CVD), a high-temperature high-pressure synthetic
process, or the like can be used as the diamond substrate 41. From
the viewpoint of controlling the operation properties of the
target, synthetic diamond having uniform physical property values
such as heat-resistance, heat conductivity, and the like is
favored. In particular, from the viewpoint of heat-resistance,
synthetic diamond prepared by a high-temperature high-pressure
synthetic process is favored.
The thickness of the diamond substrate 41 is 0.1 mm to 10 mm so
that both heat conductivity and a radiation transmission property
in the substrate thickness direction can be achieved. The diamond
substrate 41 may be composed of single crystal diamond or
polycrystal diamond. From the viewpoint of heat conductivity,
single crystal diamond is preferred. The diamond substrate 41 may
contain 2 ppm to 800 ppm of nitrogen since impact resistance is
improved and thus the portability of a radiation generating
apparatus to which the target 9 can be applied can be improved.
The target layer 42 contains a metal element having a large atomic
number, a high melting point, and a high specific gravity as a
target metal. From the viewpoint of affinity to the diamond
substrate 41, the target metal may be at least one metal selected
from the group consisting of tantalum, molybdenum, and tungsten
whose carbides exhibit negative standard free energy of formation.
The target metal may be a single metal or an alloy, or a metal
compound such as a carbide, nitride, or oxynitride of the
metal.
The thickness of the target layer 42 is within the range of 1 .mu.m
or more and 20 .mu.m or less. The lower limit and upper limit of
the range of the thickness of the target layer 42 are determined
from the viewpoints of ensuring radiation output intensity and
decreasing interfacial stress. The thickness of the target layer 42
may be in the range of 1.5 .mu.m or more and 12 .mu.m or less.
The thickness of the carbon-containing region 45 will now be
described. The thickness of the carbon-containing region 45 in the
thickness direction of the target layer 42 may be 0.005 times to
0.1 times the thickness of the target layer 42. The lower limit of
the thickness of the carbon-containing region 45 is determined
based on the carbon's stress relaxing effect and is more preferably
50 nm or more. The upper limit of the thickness of the
carbon-containing region 45 is determined from the viewpoint of
heat-resistance of the target 9 and is more preferably 500 nm or
less.
In this embodiment, the carbon-containing region 45 is a region of
the diamond substrate 41 near the surfaces of the diamond substrate
41. In this region, thermal structural changes occurred so that sp3
bonds are converted into sp2 bonds. In other words, in this
embodiment, the carbon-containing region 45 is part of the diamond
substrate 41.
In this specification, the carbon-containing region 45 refers to a
region where carbon atoms are bonded to one another through sp2
hybrid orbitals resulting from .sigma. bonds and .pi. bonds and
where carbon-carbon double bonds are present. Accordingly,
so-called one-and-a-half bonds found in .pi. electron conjugated
system and aromatic carbon compounds indicate the state in which
50% of the bonds are double bonds.
Diamond solely constituted by sp3 bonds has high elastic modulus,
high hardness, and high thermal conductivity due to its covalently
bonded cubic structure. In contrast, graphite, which is an
allotrope of diamond, has a layered hexagonal structure and
carbon-carbon bonds in each layer form sp2 hybrid orbitals. Within
each layer of graphite, carbon atoms are covalently bonded and the
bonding strength of between carbon atoms is relatively high.
However, layers are bonded to each other through Van Der Waals
force and thus bonding strength between carbon atoms of adjacent
layers is relatively weak.
Due to such a difference in structure, the Young's modulus of
diamond and the Young's modulus of graphite are 1000 GPa and 10
GPa, respectively, and differ from each other by two orders of
magnitude. Accordingly, it is possible to decrease the Young's
modulus of a portion of the diamond substrate 41 for a control
width of several to several hundred GPa by controlling the
percentage of sp3 bonds to be converted into sp2 bonds in the
diamond substrate 41.
The linear expansion coefficient can also be increased by several
times from about 1.times.10.sup.-6.degree. C..sup.-1 of diamond to
about 6.times.10.sup.-6.degree. C..sup.-1 of graphite when the
percentage of the sp3 bonds of the diamond substrate 41 to be
converted to sp2 bonds is controlled. As a result, the
carbon-containing region 45 exhibits a linear expansion coefficient
close to the target metal (for example, 4.5.times.10.sup.-6.degree.
C..sup.-1 of tungsten) than the region 46 constituted by sp3
bonds.
Accordingly, the carbon-containing region 45 in this embodiment can
be regarded as a stress relaxing region against thermal stress and
also as a matching region that reduces the mismatch of linear
expansion coefficient.
Next, the relationship between the issue the invention is directed
to and the structure of the target layer is described in detail
with reference to FIGS. 9A to 9D. The "degradation of performance
of a target in maintaining the anode potential at the target layer"
is strongly correlated with the layer structure of the target.
FIGS. 9A and 9B are respectively a schematic diagram of a typical
cross-sectional layer structure of a transmission type target 69
and a schematic diagram of a typical cross-sectional layer
structure of a reflection target 89. FIGS. 9C and 9D respectively
show a distribution of microcracks 68 in the transmission type
target 69 and a distribution of microcracks 68 in the reflection
target 89.
As shown in FIG. 9B, in the reflection target 89 that includes a
target layer 82 and a substrate 81, radiation generated at the
electron penetration depth dp is not output from the opposite side
of the target layer 82 and is output in a rear direction 83 from
the surface on which the electron beam 5 is incident. Accordingly,
the thickness t of the target layer 82 of the reflection target 89
can be set to a sufficiently large value relative to the electron
penetration depth dp without considering the radiation
transmittance in the layer thickness direction.
To be more specific, the electron penetration depth dp, which is
dependent on tube voltage, is typically within the range of several
micrometers to less than 20 micrometers. The thickness t of the
target layer 82 is typically within the range of several
millimeters to less than 20 millimeters due to thermal capacity
design and strength design requirements, etc. The thickness of a
heat-generating portion of the target layer 82 substantially
coincides with the electron penetration depth dp of the electron
beam 5 relative to the target layer 82. Accordingly, the thickness
of the heat-generating portion of the reflection target 89 is
sufficiently small relative to the thickness t of the target layer
82.
Accordingly, the thermal stress generated in the reflection target
89 is concentrated near the surface layer of the target layer 82.
The possibility that a microcrack 68 would penetrate entirely
through the target layer 82 in the thickness direction is low.
Furthermore, due to design flexibility of the reflection target 89,
a supporting member having electrical conductivity such as copper
can be disposed on the rear surface of the target layer 82.
Accordingly, degradation of the performance of the target in
maintaining the anode potential at the target layer 82 rarely
occurs in the reflection target 89.
In contrast, as shown in FIG. 9A, radiation generated at an
electron penetration depth dp in the target layer 62 of the
transmission type target 69 passes through the target layer 62 and
the diamond substrate 61 and is output in a front direction 84.
Accordingly, the thickness t of the target layer 62 of the
transmission type target 69 is set to a thickness substantially
equal to the electron penetration depth dp (0.5.times.dp or more
and 1.5.times.dp or less) while taking into consideration
attenuation in the target layer 62.
Accordingly, thermal stress generated in the transmission type
target 69 is distributed over the entire thickness t of the target
layer 82. Thus, as shown in FIG. 9C, a microcrack 68 that
penetrates entirely through the layer of the target layer 82 in the
thickness direction may occur. In such a case, unlike in the case
of the reflection target 89, the performance of the target in
maintaining the anode potential at the target layer 62 is degraded
due to the microcrack 68 since it is difficult to supply the anode
potential from the rear side of the target layer 62.
As discussed above, the phenomenon of "degradation of performance
of the target in maintaining the anode potential at the target
layer" due to occurrence of the microcrack 68 is an issue that is
deeply related to the structure of a transmission type target.
Next, thoughts of the inventors regarding the mechanism by which a
microcrack 68 propagates inside the target layer 62 and comes to
cause formation of island regions 65 and 65' are discussed.
A first factor responsible for generation of a microcrack 68 in the
target layer 62 is thermal stress. Thermal stress is defined by
mismatch in linear expansion coefficient between the target layer
62 and the diamond substrate 61
(.DELTA..alpha.=.alpha.62-.alpha.61, 5 to 9.times.10.sup.-6.degree.
C..sup.-1) and the temperature increase (650.degree. C. to
1400.degree. C.) during operation of the transmission type
target.
The thermal stress has a vector in a direction parallel to the
layer surface of the target layer 62 and serves as a driving force
responsible for causing separation of and generating cracks in the
target layer. This is qualitatively disclosed in Japanese Patent
Laid-Open No. 2002-298772.
A second factor responsible for generation of the microcrack 68 in
the target layer 62 is that the diamond substrate 61 has a high
elastic modulus.
Diamond has a particularly high elastic modulus (1050 GPa at room
temperature (25.degree. C.)) due to its unique crystal structure.
The elastic modulus of a high-melting-point metal element selected
as the target metal is not as high as that of diamond. Thus,
thermal stress in the target tends to be concentrated in the target
layer 62.
The elastic modulus (Young's modulus) of representative examples of
metal elements that can be used as the target metal is 403 for
tungsten, 327 for molybdenum, and 181 for tantalum in terms of GPa
at room temperature, 25.degree. C.
A third factor responsible for generation of a microcrack 68 in the
target layer 62 is that the target layer 62 has a polycrystal
structure.
A gas-phase deposition process (dry film-forming process) such as
sputtering, vapor deposition, or chemical vapor deposition (CVD) is
generally employed to form the target layer 62. A target layer 62
formed by a gas-phase deposition process frequently takes a
columnar structure with columnar crystal grains extending in the
layer thickness direction. Crystal grain boundaries contained in
the columnar structure intersect the direction in which thermal
stress acts. Accordingly, the crystal grains contained in the
columnar structure are a factor that limits the mechanical stress
of a shear failure mode of the target layer 62.
A fourth factor responsible for generation of the microcrack 68 in
the target layer 62 is that the crystal grains constituting the
target layer 62 become more coarse as they undergo an increase in
temperature.
FIG. 8A is a conceptual rendering of a grain boundary energy
distribution of a polycrystal structure constituting a target layer
immediately after completion of deposition. The vertical axis
indicates grain boundary energy per unit grain boundary area and
the horizontal axis indicates the positions of crystal grains G1 to
G9 in the polycrystal structure of the target layer.
The positions of ten peaks found in this grain boundary energy
distribution correspond to crystal grain boundaries and the
peak-to-peak distance .DELTA.x of the energy distribution indicates
a crystal grain diameter of each of the crystal grains G1 to G9. At
this initial stage, no significant variation is observed in terms
of crystal grain diameter and grain boundary energy and no
microcracks are present in the target layer.
FIG. 8B is a conceptual rendering of a grain boundary energy
distribution of the target layer that underwent heat history
accompanying radiation generating operation. The distribution
corresponds to crystal grains G1 to G4 and G6 to G9. FIG. 8B shows
that both crystal grains that are growing and crystal grains that
are shrinking are present at this intermediate stage. The crystal
grain G5 which was present in the target layer in the initial stage
merges into the crystal grain G4 that is growing, and disappears.
The grain boundary energy at the grain boundary defined by a
growing crystal grain whose grain diameter is increasing is high
compared to the initial stage. At this stage, no microcracks are
found in the target layer.
FIG. 8C is a conceptual rendering of the grain boundary energy
distribution corresponding to the crystal grains G2, G4, and G8
that have become more coarse as a result of merging with adjacent
crystal grains. FIG. 8C shows that selective coarsening of crystal
grains and the increase in grain boundary energy are more notable
in this later stage than in the intermediate stage. The increase in
grain boundary energy is presumably caused by grains taking in the
energy of dislocations, microdefects, etc., at grain boundaries of
fine crystal grains that have been merged and disappeared.
Presumably, a microcrack occurs and the grain boundary energy is
released as the grain boundary energy is increased and comes to
exceed the upper limit E.sub.th up to which a continuous film
structure can be maintained.
As discussed above, the mechanism of radiation output fluctuation
observed in the aforementioned study results have all of the first
to fourth factors correlated to one another although the detailed
mechanism is not clear. It is presumed that because of this
mechanism, a microcrack 68 that extends in the in-plane direction
and the thickness direction of the target layer 62 occurred as
shown by the transmission type target unit 71 in FIGS. 10A and
10B.
The stress relaxing effect exhibited by the carbon-containing
region of this embodiment presumably suppresses generation of a
microcrack in the target layer that has undergone heat history
because the carbon-containing region apparently increases the upper
limit E.sub.th of the grain boundary energy as shown in FIG.
8C.
The carbon-containing region not only has a stress relaxing effect
but also has electrical conductivity attributable to the .pi.
electron conjugated system since a particular concentration of sp2
bonds are contained. Thus, the carbon-containing region also has an
effect of ensuring the electrical connection to the target layer.
Note that from the viewpoint of the electrical conductivity
attributable to the .pi. electron conjugated system, 20% or more of
the carbon-carbon bonds are preferably sp2 bonds and more
preferably 40% or more of the carbon-carbon bonds are sp2
bonds.
Since the carbon-containing region 45 has electrical conductivity,
the electrical connection between the island region 65 and the
anode member 49 can be ensured in the layer surface direction in
the event of occurrence of a microcrack 68 in the target layer 62
shown in FIGS. 10A and 10B.
Since the diamond substrate 41-side of the target layer 42 has a
portion that is connected to the carbon-containing region 45
containing sp2 bonds, a target 9 in which the target layer 42 is
stably maintained at the anode potential can be offered. In other
words, since the carbon-containing region 45 is positioned between
the target layer 42 and the region 46 of the diamond substrate 41,
a target 9 in which the target layer 42 is stably maintained at the
anode potential can be offered.
The material constituting the carbon-containing region 45 may be a
carbon compound that has sp2 bonds in a cyclic main chain, a linear
main chain, or a three-dimensional network main chain or a diamond
allotrope having sp2 bonds.
Typical materials used as the allotrope of diamond are graphite,
graphene, and glassy carbon solely constituted by sp2 bonds. These
materials can be used to form the carbon-containing region 45.
However, the material for the carbon-containing region 45 need not
be solely formed of sp2 bonds.
Examples of the material for the carbon-containing region 45
include amorphous carbon having dangling bonds, carbon nanotubes
mainly constituted by six-membered rings, and fullerene constituted
by five-membered rings and six-membered rings. Other examples of
the material constituting the carbon-containing region 45 include
graphite nanofibers in which fibrous graphene sheets are stacked
and diamond-like carbon having three dimensional carbon-carbon
atomic network containing sp3 bonds and sp2 bonds.
The carbon compound used as the material for the carbon-containing
region 45 may be a hydrocarbon compound having functional groups
introduced into an allotrope of diamond, a polymer complex having
coordinate bonds with a particular metal ion, or an electrically
conductive polymer having .pi. electron conjugated system developed
on a main chain as long as sp2 bonds are in the main chain.
Next, a basic production method of the first embodiment for
producing the target 9 shown in FIG. 1A is described with reference
to FIGS. 5A-1 to 5A-3.
The production method of the first embodiment includes the
following steps.
First, as shown in FIG. 5A-1, a diamond substrate 41 is prepared.
The diamond substrate 41 is a polyhedron. Next, as shown in FIG.
5A-2, the diamond substrate 41 is heat-treated in a deoxidizing
atmosphere to convert part of the diamond substrate 41 into
graphite or the like. In other words, in the heating step of this
embodiment, some of sp3 bonds contained in the diamond substrate 41
are structurally changed by applying heat and converted into sp2
bonds.
Next, as shown in FIG. 5A-3, a target layer 42 containing a target
metal is formed on the diamond substrate 41 having a
carbon-containing region 45 so as to form a target 9.
The deoxidizing atmosphere employed in this embodiment has a
technical significance of suppressing a volume decrease and loss of
the diamond substrate 41 resulting from combustion of the diamond
substrate 41. The deoxidizing atmosphere in the heating step is
created by filling the interior of a process chamber with an inert
gas such as nitrogen or rare gas or by purging oxygen from the
process chamber through evacuation. The heating step may be
performed in a vacuum atmosphere or an inert gas atmosphere.
The heating step in this embodiment may be performed at a
temperature in the range of 650.degree. C. or more and 2000.degree.
C. or less from the viewpoints of process time and retention of
strength of the diamond substrate. The heating step may be
performed while bringing the diamond substrate 41 in contact with a
metal stage having high heat conductivity or while thermally
insulating and supporting the diamond substrate 41 with a jig
composed of a porous ceramic having low heat conductivity.
The carbon-containing region 45 may be distributed over the entire
diamond substrate 41 but is preferably at least present at high
concentration at the surface on which the target layer 42 is to be
formed.
The inventors have conducted investigations and found that a
carbon-containing region 45 is preferentially formed at the
surfaces of the diamond substrate 41 as shown in FIG. 5A-2 even
when the diamond substrate 41 is uniformly heated in a deoxidizing
atmosphere. This is presumably because the number of defects is
larger at the surfaces of the diamond substrate 41 than in the
inner side of the substrate and thus sp3 bonds in the surface-side
portions are preferentially converted into sp2 bonds compared to
the inner side.
Modification examples of the production method of the first
embodiment will now be described with reference to FIGS. 5B-1 to
5B-3 and FIGS. 5C-1 and 5C-2.
The production method illustrated in FIGS. 5B-1 to 5B-3 differs
from the production method illustrated in FIGS. 5A-1 to 5A-3 in
that a metal-containing layer forming step (FIG. 5B-2) of forming a
metal-containing layer 72 containing a target metal on the diamond
substrate 41 is performed before the heating step (FIG. 5A-3). This
production method has two advantages over the production method
shown in FIGS. 5A-1 to 5A-3.
The first advantage is that the metal-containing layer 72 has a
larger absorbance index in an infrared wavelength region than the
diamond substrate 41 and heat conductivity lower than that of the
diamond substrate. Accordingly, the metal-containing layer 72 is
selectively heated during the heat treatment. As shown in FIG.
5B-3, the carbon-containing region 45 is preferentially formed at
the target layer 42-side surface of the diamond substrate 41
compared to the interior and other surfaces of the diamond
substrate 41. As a result, the quantity of heat needed to form the
carbon-containing region 45 can be decreased, contributing to
energy conservation.
The second advantage is that the metal material constituting the
metal-containing layer 72 is supplied with carbon as carbon atoms
contained in the diamond substrate 41 diffuse during the heating
step shown in FIG. 5B-3. The carbon atoms contained in the diamond
substrate 41 diffuse into the metal-containing layer 72 since there
is a carbon concentration gradient from the diamond substrate 41
toward the metal-containing layer 72. Diffusion of the carbon atoms
into the metal-containing layer 72 continues until a concentration
gradient in a thermal equilibrium state is reached.
During the elementary process of carbon diffusion, sp3 bonds
constituting the diamond substrate 41 and being present at the
surface in contact with the metal-containing layer 72 are unbonded
and carbon atoms are supplied to the metal-containing layer 72.
Since the diamond substrate 41 supplies carbon atoms to the
metal-containing layer 72, the dangling bonds generated by
consumption of carbon atoms form sp2 bonds.
The production method illustrated in FIGS. 5C-1 and 5C-2 differs
from the production method illustrated in FIGS. 5A-1 to 5A-3 in
that the diamond substrate 41 is subjected to a heating step while
performing the step of forming a metal-containing target layer 42
on the diamond substrate 41. In this embodiment also, it is
possible to preferentially form the carbon-containing region 45 at
the target layer 42-side of the diamond substrate 41. This
production process has the first and second advantages described
above over the production method illustrated in FIGS. 5A-1 to 5A-3
and also a third advantage of requiring fewer steps.
Modification examples of the target of the first embodiment will
now be described with reference to FIGS. 4B and 4C.
The modification example shown in FIG. 4B differs from the first
embodiment in that the carbon-containing region 45 is discretely
formed between the target layer 42 and the region 46 constituted by
sp3 bonds. The carbon-containing region 45 may be a continuous
layer or a discontinuous layer or may be even dispersed in the
diamond substrate 41 without forming a particular layer as long as
sp2 bonds are contained in the target layer 42-side portion of the
substrate.
The target 9 shown in FIG. 4B can be formed by irradiating the
metal-containing layer 72 with an infrared laser beam in the steps
shown in FIGS. 5B-2 and 5B-3, for example.
In the modification example illustrated in FIG. 4C, a
carbon-containing region is formed as a carbon-containing layer 47
between the target layer 42 and the diamond substrate 41. In this
modification example also, the carbon-containing layer 47 may be a
continuous layer or a discontinuous layer as long as sp2 bonds are
contained on the target layer 42-side. In this specification, the
modification example illustrated in FIG. 4C is referred to as a
target of a second embodiment.
An example of a method for producing the target 9 according to the
second embodiment illustrated in FIG. 4C is illustrated in FIGS.
6A-1 to 6A-3.
First, as shown in FIG. 6A-1, a polyhedral diamond substrate 41 is
prepared. Then as shown in FIG. 6A-2, a carbon-containing layer 47
containing sp2 bonds such as graphite or glassy carbon is formed on
one surface of the diamond substrate 41. Then as shown in FIG.
6A-3, a target layer 42 is formed on the carbon-containing layer
47. Thus, a target 9 of the second embodiment having the
carbon-containing layer 47 as an intermediate layer can be
produced.
Next, a first modification example of the method for producing the
target 9 according to the second embodiment is described with
reference to FIGS. 6B-1 to 6B-4.
The production method of the first modification example differs
from the production method illustrated in FIG. 6A-1 to 6A-3 in that
a carbon-containing film 77 which may contain sp2 bonds or no sp2
bonds is formed and used as a starting material, and a heat
treatment is performed to increase the concentration of the sp2
bonds so as to form a carbon-containing layer 47.
In particular, first, as illustrated in FIG. 6B-1, a polyhedral
diamond substrate 41 is prepared. Then in a step shown in FIG.
6B-2, a carbon-containing film 77 is formed on the diamond
substrate 41. In a step shown in FIG. 6B-3, at least the
carbon-containing film 77 is subjected to a heat treatment. Then in
a step shown in FIG. 6B-4, a target layer 42 is formed on the
carbon-containing layer 47. As a result, a target 9 of the second
embodiment that includes the carbon-containing layer 47 as an
intermediate layer is obtained.
A second modification example of the method for producing the
target 9 of the second embodiment will now be described with
reference to FIGS. 6C-1 to 6C-4.
The production method of the second modification example differs
from the production method illustrated in FIGS. 6B-1 to 6B-4 in
that a step of converting a carbon-containing film 77 into a
carbon-containing layer 47 illustrated in FIG. 6C-4 is performed
after a step of forming a target layer 42 on the carbon-containing
film 77 illustrated in FIG. 6C-3.
A third modification example of the method for producing the target
9 of the second embodiment will now be described with reference to
FIGS. 6D-1 to 6D-4.
The production method of the third modification example differs
from the production method illustrated in FIGS. 6C-1 to 6C-4 in
that a metal-containing layer 72 containing a target metal is
formed as shown in FIG. 6D-3 and then a step of converting a
carbon-containing film 77 into a carbon-containing layer 47 by a
heat treatment and a step of converting the metal-containing layer
72 into a target layer 42 are performed simultaneously as shown in
FIG. 6D-4.
The basic method for producing the target 9 of the second
embodiment illustrated in FIGS. 8A-1 to 6A-3 is advantageous over
the first to third modification examples in that the method
involves fewer steps. The second and third modification examples
are advantageous over the basic method and the first modification
example in that the second and third advantages described above are
achieved because the carbon-containing layer 47 containing sp2
bonds is formed by a heat treatment after formation of the target
layer 42 or the metal-containing layer 72.
The target of the second embodiment is disadvantageous in terms of
production compared to the target of the first embodiment in that a
step of forming an independent intermediate layer is necessary.
However, the target of the second embodiment is advantageous in
terms of production compared to the target of the first embodiment
in that there is no need to perform a high-temperature treatment in
a deoxidizing atmosphere needed to convert the diamond substrate
41. Whether to choose the first embodiment or the second embodiment
can be appropriately determined by considering the consistency with
other production steps.
As shown in FIGS. 5A-1 to 5E-3 and FIGS. 6A-1 to 6D-4, in both the
first embodiment and the second embodiment, the target production
method includes a target layer formation step of forming a target
layer 42 on one surface of a diamond substrate 41. The target
production method also includes a sp2 bond formation step of
forming a carbon-containing region 45 having sp2 bonds and being in
contact with a diamond substrate 41-side of the target layer
42.
As shown in FIGS. 5A-1 to 5E-3, the target layer formation step of
the target production method of the first embodiment includes a
step of forming a metal layer (the target layer 42 or
metal-containing layer 72) containing a target metal on one surface
of a diamond substrate 41. The steps illustrated in FIGS. 5A-3,
5B-2, 5C-2, 5D-2, and 5E-3 each correspond to the step of forming a
metal layer (the target layer 42 or metal-containing layer 72).
Moreover, as shown in FIGS. 5A-1 to 5E-3, the sp2 bond formation
step of the target production method of the first embodiment
includes a heating step of heating the diamond substrate and
converting at least some of sp3 bonds contained in the surface of
the diamond substrate into sp2 bonds. The steps illustrated in
FIGS. 5A-2, 5B-3, 5C-2, 5D-3, and 5E-2 each correspond to the
heating step.
As shown in FIGS. 6A-1 to 6A-3, the sp2 bond formation step of the
target production method of the second embodiment includes forming
a carbon-containing layer 47 containing sp2 bonds on one surface of
the diamond substrate 41. The step of forming the carbon-containing
layer 47 containing sp2 bonds corresponds to the step illustrated
in FIG. 6A-2.
As shown in FIGS. 6B-1 to 6D-4, the sp2 bond formation step of the
target production method of the second embodiment includes a step
of forming a carbon-containing film 77 having sp3 bonds on one
surface of a diamond substrate 41 and a step of heating at least
the carbon-containing film 77 so as to convert the
carbon-containing film 77 into a carbon-containing layer 47 having
sp2 bonds.
Steps illustrated in FIGS. 6B-1, 6C-2, and 6D-2 each correspond to
the step of forming the carbon-containing film 77 having sp3 bonds.
Steps illustrated in FIGS. 6B-3, 6C-4, and 6D-4 each correspond to
the step of forming the carbon-containing film 77 into the
carbon-containing layer 47 having sp2 bonds.
Next, an embodiment in which a target 9 is used in a target unit 51
mounted as an anode into a radiation generating apparatus 101 shown
in FIG. 3A is described with reference to FIGS. 4D and 4E.
FIG. 4D shows a transmission type target unit 51 (hereinafter
simply referred to as a target unit 51) equipped with the target 9
shown in FIG. 4A in a hollow portion of a cylindrical anode member
49. The inner peripheral portion of the hollow portion of the
target unit 51 is connected to the outer peripheral portion of the
target 9 via brazing 48. The brazing 48 may be an alloy having a
low melting point such as an alloy containing tin, silver, or the
like. In this embodiment, in the outer peripheral portion of the
target 9, the periphery of the diamond substrate 41 overlaps the
periphery of the target layer 42.
In the target unit 51, the brazing 48 serves as a bonding material
that holds the target 9 and is responsible for establishing an
electrical connection between the anode member 49 and the target
layer 42.
FIG. 4E is a cross-sectional view of the target unit 51 shown in
the plan view of FIG. 4D taken along line IVE-IVE in FIG. 4D.
According to the structure of this embodiment, the sp2 bonds
exhibit electric conductivity and the reliability of the electrical
connection between the target 9 and the anode member 49 is
enhanced.
The anode member 49 is composed of a material having a large
specific gravity. Thus the anode member 49 can define radiation
output angle (radiation angle) in a required direction and can
block the radiation from going out in undesirable directions as
shown in FIG. 3A.
From the viewpoint of further size reduction, a metal element
having a specific absorption edge energy may be appropriately
selected as the material for forming the anode member 49 on the
basis of a characteristic X-ray energy of the radiation generated
from the target layer 42.
Specific examples of such a metal element for forming the anode
member 49 include copper, silver, molybdenum, tantalum, tungsten,
KOVAR (US registered trademark, Ni--Co--Fe alloy produced by CRS
Holdings Inc.), MONEL (Ni--Cu--Fe alloy, US registered trademark
co-owned by Special Metals Corporation and Huntington Alloys
Corporation), and stainless steel. The same metal element as the
target metal contained in the target layer 42 may also be contained
in the anode member 49.
The scope of the invention disclosed in this specification also
encompasses a multilayered target layer 42 formed of two or more
layers and an embodiment in which the carbon-containing region has
sp1 bonds (carbon-carbon triple bonds) as long as the effect
brought by the carbon-containing region having sp2 bonds is
obtained.
EXAMPLES
A radiation generating apparatus equipped with a target according
to an embodiment of the invention was prepared by the procedure
described below, and operated to evaluate output stability.
Example 1
FIG. 1A is a schematic diagram of a target 9 prepared in Example 1.
The procedure of preparing the target 9 of Example 1 is shown in
FIGS. 5B-1 to 5B-4. A section specimen 55 of the target 9 of this
example is shown in FIG. 2A and analyzed regions 145 and 146 in the
section specimen 55 are shown in FIG. 2B. FIG. 2C shows a profile
of the results of electron energy loss spectroscopy. FIG. 2D shows
a calibration graph used to identify the normalized sp2 bond
concentration C.sub.sp2.
FIG. 3A shows a schematic structure of a radiation generating tube
102 loaded with the target 9 of this example. FIG. 3B shows a
radiation generating apparatus 101 loaded with the radiation
generating tube 102. The evaluation system used to evaluate
stability of radiation output of the radiation generating apparatus
101 of this example is shown in FIG. 7.
First, as shown in FIG. 5B-1, a diamond substrate 41 having a
diameter of 6 mm and a thickness of 1 mm and composed of single
crystal diamond was prepared. The diamond substrate 41 was cleaned
with a UV ozone asking device to remove residual organic matter on
the surfaces of the diamond substrate 41.
Next, as shown in FIG. 5B-2, a metal-containing layer 72 having a
thickness of 5 .mu.m and composed of tungsten was sputter-deposited
on one of the clean surfaces of the diamond substrate 41 by using
an argon gas as a carrier gas and a sintered tungsten target as a
sputtering target.
Then a stack of the metal-containing layer 72 and the diamond
substrate 41 was placed in an image furnace (not shown) by using a
ceramic holder jig (not shown) composed of alumina. Then a vacuum
atmosphere was created in the interior of the image furnace. The
stack was irradiated with an infrared ray for 10 hours so that the
temperature of the stack was 1300.degree. C. so as to conduct a
heating step. As a result, a target 9 of Example 1 was
obtained.
The thickness of the target layer 42 of the target 9 came out to be
6 .mu.m.
Visual observation of the target 9 of this example revealed that
brown to black shaded regions were distributed mainly near the
surfaces of the diamond substrate 41, as shown in FIG. 5B-3.
Next, as shown in FIG. 2A, a section specimen 55 was cut out from
the target 9 by mechanical polishing and focused ion beam (FIB)
processing. The section specimen 55 was taken to include a region
that extended from the lower edge of the target layer 42 by 300 nm
toward the target layer 42 side and a region that extended from the
lower edge of the target layer 42 by 500 nm toward the diamond
substrate side.
A region near the border between the target layer 42 and the
diamond substrate in the section specimen 55 was observed with a
scanning transmission electron microscope (STEM). From the image
contrast, a region constituted by elements having a specific
gravity smaller than that of the target layer 42 was found near the
target layer 42 but within the diamond substrate 41. This region
was assumed to be the carbon-containing region 45.
The assumed carbon-containing region 45 exhibited a halo pattern
when measured with an electron beam diffractometer (STEM-ED)
attached to STEM and no lattice fringes were observed in a
high-resolution observation mode, thereby revealing that the
carbon-containing region 45 was an amorphous phase. EDX analysis
attached to STEM found that the carbon-containing region 45 is a
region mainly composed of carbon.
In order to identify the carbon-containing region, which is a
feature of the invention, STEM-EELS evaluation was performed by
using an electron loss energy spectrometer attached to STEM.
FIG. 2C shows the EELS profile obtained by the STEM-EELS analysis.
The horizontal axis indicates the electron loss energy and the
vertical axis indicates the intensity I of an EELS signal. The
signal intensity I.sub.285 at an electron loss energy of 285 eV
corresponds to the concentration of .pi. bonds. The signal
intensity I.sub.292 at an electron loss energy of 292 eV
corresponds to the concentration of .sigma. bonds.
In EELS analysis, the sp2 bonds which are carbon-carbon double
bonds are detected as the EELS signal at 285 eV and the EELS signal
at 292 eV (I.sub.285 and I.sub.292) attributable to .sigma. bonds
and .pi. bonds. The sp3 bonds which are carbon-carbon single bonds
are detected as the EELS signal (I.sub.292) at 292 eV attributable
to .sigma. bonds.
It can be qualitatively understood from the profile shown in FIG.
2C that in the analyzed region 145, the concentration of sp2 bonds
constituted by .pi. bonds and .sigma. bonds is significantly high
and, in the analyzed region 146, the concentration of sp3 bonds
constituted by .sigma. bonds is significantly high.
An EELS signal was observed at 285 eV in the analyzed region 146.
However this EELS signal is presumably attributable to unavoidable
hydrocarbons that had been physically adsorbed from the analytic
atmosphere or polymeric carbon resulting from irradiation with the
electron beam. In other words, it is assumed that the EELS signal
detected at 285 eV possibly indicates real carbon bonds of the
specimen superimposed with background signals irrelevant to the
specimen.
In order to confirm the influence of the background signals,
synthetic diamond not subjected to a heat treatment was analyzed as
a reference specimen through EELS. The EELS signal having about the
same intensity as that detected in the analyzed region 146 was
detected at 285 eV. It was confirmed that background signals (noise
component) inherent to the measurement system are dominant in the
EELS signals detected at 285 eV from the analytical position 146
and the diamond reference specimen.
In general, the detection sensitivity differs for every
characteristic energy of electron loss energy. In particular,
depending on conditions inherent to the measurement system and
conditions of measurement, .pi. bond concentration/I.sub.285 signal
intensity (=.pi. bond detection sensitivity) does not match .sigma.
bond concentration/I.sub.292 signal intensity (=.sigma. bond
detection sensitivity). The EELS profile shown in FIG. 2C is raw
data and at least affected by this.
In order to eliminate the influence of these errors related to
identification of the sp2 bond concentration, the inventors of the
present application performed correction as below so as to
eliminate the influence of background signals and the dependency of
the detection sensitivity on the characteristic energy so as to
identify the value of sp2 bond concentration.
To be more specific, single crystal synthetic diamond not subjected
to a heat treatment and single crystal graphite (highly ordered
pyrolytic graphite or HOPG) not subjected to a heat treatment were
used as the reference specimens and a calibration line shown in
FIG. 2D was drawn by using the two reference specimens to eliminate
the error caused by background signals.
The error attributable to the dependency of the detection
sensitivity on characteristic energy was eliminated by using the
EELS signal intensity ratio I.sub.285/I.sub.292, which is the ratio
obtained by dividing the EELS signal intensity I.sub.285 detected
at 285 eV by the EELS signal intensity I.sub.292 detected at 292
eV.
Then normalized sp2 bond concentration C.sub.sp2 obtained by
eliminating the aforementioned two types of errors was defined by
using formula (1) shown below.
.times..times..times..times..times..times..function..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times..times..times..times..times..times..time-
s..times. ##EQU00001##
As a result, it was found that the normalized sp2 bond
concentrations in the analyzed region 145 and the analyzed region
146 were 98.6% and 0.8%, respectively.
As discussed above, based on the results of the EELS analysis of
the diamond substrate 41 of the target 9 of this example and other
composition-structure analysis, the analyzed region 145 was
identified to be composed of amorphous carbon mainly containing sp2
bonds and the analyzed region 146 was identified to be composed of
diamond mainly composed of sp3 bonds.
The analyzed regions 145 and 146 were set so that they respectively
lay in the assumed carbon-containing region 45 and the region 46
from which lattice fringes caused by crystallinity of diamond were
observed in a high resolution observation mode as shown in FIG. 2B.
STEM-EELS line analysis conducted in the thickness direction of the
target layer 42 revealed that the assumed carbon-containing region
45 was distributed by having a thickness of 80 nm to 250 nm. The
STEM-EELS line analysis was performed at 20 nm intervals and the
detection intervals were appropriately decreased to several
nanometers in the area near the border between the assumed
carbon-containing region 45 and the other structure.
Next, a radiation generating tube 102 equipped with the target 9
prepared in Example 1 was produced by the following procedure.
First, as shown in FIGS. 4D and 4E, the target 9 was brazed onto an
anode member 49 composed of copper to form a target unit 51 that
served as an anode. Then an electron emitting source 3 including an
impregnated type electron gun equipped with an electron emitting
portion 2 composed of lanthanum boride (LaB.sub.6) was brazed onto
a cathode member (not shown) composed of Kovar so as to prepare a
cathode.
Then the cathode and the anode were respectively brazed onto two
openings of an insulation tube 110 composed of alumina (FIG. 3A) so
as to form an envelope. An inner space 13 of the envelope was
evacuated with an evacuator (not shown) until a degree of vacuum of
1.times.10.sup.-6 Pa was reached. As a result, a radiation
generating tube 102 shown in FIG. 3A was produced.
A drive circuit 103 was electrically connected to the cathode and
the anode of the radiation generating tube 102. The radiation
generating tube 102 and the drive circuit 103 were housed in an
extra space 43 of a container 120. As a result, a radiation
generating apparatus 101 shown in FIG. 3B was obtained.
An evaluation system 70 shown in FIG. 7 was prepared to evaluate
the drive stability of the radiation generating apparatus 101. In
the evaluation system 70, a dosimeter 26 was placed in front of a
radiation transmitting window 121 of the radiation generating
apparatus 101 and disposed at a position 1 m from the radiation
transmitting window 121. The dosimeter 26 was connected to the
drive circuit 103 via a measurement control unit 207 and was
capable of measuring the intensity of the output radiation from the
radiation generating apparatus 101.
The drive conditions for evaluation of drive stability were as
follows. The tube voltage of the radiation generating tube 102 was
+100 kV, the current density of the electron beam irradiating the
target layer 42 was 4 mA/mm.sup.2, and pulsed drive of repeating an
electron irradiation period of 2 seconds and a non-irradiation
period of 198 seconds was performed. An average value of output
intensity was determined from the intensities detected during 1
second at the middle of the electron irradiation period and the
obtained average value was assumed to be the detected radiation
output intensity.
The stability evaluation of the radiation output intensity was
performed in terms of a retention rate determined by normalizing
the radiation output intensity after elapse of 100 hours from the
start of the radiation output by using the initial radiation output
intensity.
In evaluating the stability of the radiation output intensity, the
tube current flowing from the target layer 42 to a ground electrode
16 was measured and constant-current control was performed by using
a negative feedback circuit (not shown) so that the fluctuation in
the electron current density irradiating the target layer 42 was
within 1%. During evaluation of drive stability of the radiation
generating apparatus 101, a discharge counter 76 monitored that the
radiation generating apparatus 101 was driven stably without
causing discharge.
The retention rate of the radiation output of the radiation
generating apparatus 101 of this example was 0.98. This confirmed
that the radiation generating apparatus 101 equipped with the
target 9 of this example does not show significant fluctuations in
radiation output even after long hours of driving history and
stable radiation output intensity is obtained. The radiation
generating apparatus 101 of this example that underwent the test of
evaluating stability of radiation output intensity was disassembled
to take out the target unit 51. No microcrack were observed in the
target layer 42 of the target unit 51.
Example 2
In Example 2, a radiation generating apparatus 101 was produced as
in Example 1 except that the steps shown in FIGS. 5A-1 to 5A-3 were
performed to prepare a target 9. The stability of radiation output
of the radiation generating apparatus 101 was evaluated.
First, as in Example 1, surfaces of a diamond substrate 41 were
washed as shown in FIG. 5A-1. Then as shown in FIG. 5A-2, the
diamond substrate 41 was placed in an image furnace (not shown) by
using a ceramic holder jig (not shown) composed of alumina. Then a
vacuum atmosphere was created in the interior of the image furnace.
The diamond substrate 41 was irradiated with an infrared ray for 10
hours so that the temperature of the diamond substrate 41 was
1500.degree. C. so as to conduct a heating step.
Next, a target layer 42 composed of tungsten and having a thickness
of 7 .mu.m was sputter-deposited on one of the surfaces of the
diamond substrate 41 that underwent the heat treatment as shown in
FIG. 5A-3. As a result, a target 9 of Example 2 was obtained.
Visual observation of this example revealed that grown to black
shaded regions were distributed mainly near the surfaces of the
diamond substrate 41, as shown in FIG. 5A-3.
As in Example 1, a section specimen 55 was cut out from the target
9 of Example 2 by mechanical polishing and FIB processing. The
section specimen 55 was taken to include a region that extended
from the lower edge of the target layer 42 by 300 nm toward the
target layer 42 side and a region that extended from the lower edge
of the target layer 42 by 500 nm toward the diamond substrate side
as shown in FIG. 2B.
A region near the border between the target layer 42 and the
diamond substrate in the section specimen 55 was observed with a
scanning transmission electron microscope (STEM). From the image
contrast, a region constituted by elements having a specific
gravity smaller than that of the target layer 42 was found near the
target layer 42 but within the diamond substrate 41. This region
was assumed to be a carbon-containing region 45. STEM-EELS line
analysis of the target layer 42 was performed in the thickness
direction and it was confirmed that the assumed carbon-containing
region 45 was distributed by having a thickness of 100 nm to 210
nm.
Next, STEM-EELS evaluation was performed by using an electron loss
energy spectrometer attached to STEM in order to identify the
carbon-containing region featured in the present invention.
As a result, it was found that the normalized sp2 bond
concentration was 95% in the detection region corresponding to the
carbon-containing region and that the normalized sp2 bond
concentration was 1% in the detection region corresponding to the
diamond substrate 41.
A radiation generating tube 102 and a radiation generating
apparatus 101 were produced as in Example 1 but by using the target
9 prepared in Example 2. The radiation generating apparatus 101 was
loaded into the evaluation system 70 for measuring the drive
stability shown in FIG. 7.
The radiation output retention rate of the radiation generating
apparatus 101 of this example was 0.97. This confirmed that the
radiation generating apparatus 101 equipped with the target 9 of
this example does not show significant fluctuations in radiation
output even after long hours of driving history and stable
radiation output intensity is obtained. The radiation generating
apparatus 101 of this example that underwent the test of evaluating
stability of radiation output intensity was disassembled to take
out the target unit 51. No microcrack were observed in the target
layer 42 of the target unit 51.
Example 3
A radiation generating apparatus 101 was produced as in Example 1
except that the steps illustrated in FIGS. 5D-1 to 5D-3 were
performed to prepare a target 9 in Example 3. The radiation output
stability of the radiation generating apparatus 101 was
evaluated.
First, as in Example 1, surfaces of a diamond substrate 41 were
washed as shown in FIG. 5D-1. Then as shown in FIG. 5D-2, a target
layer 42 composed of tungsten and having a thickness of 7 .mu.m was
sputter-deposited on one of the surfaces of the diamond substrate
41 so as to form a stack.
Next, the stack was placed in a chamber (not shown) and the
interior of the chamber was purged with nitrogen gas. The stack's
surface on which the target layer 42 was formed was irradiated with
an infrared ray having a wavelength of 808 nm via a quartz window
of the chamber by using a semiconductor laser beam source.
Irradiation with the laser beam was performed 1000 times by pulsed
driving using a Q switch. As a result, a target 9 in which a
portion of the diamond substrate 41 near the interface with the
target layer 42 turned brown to black in color was obtained as
shown in FIG. 5D-3.
As shown in FIG. 2A, a section specimen 55 was cut out from the
target 9 of Example 3 by mechanical polishing and FIB processing.
The section specimen 55 was taken to include a region that extended
from the lower edge of the target layer 42 by 300 nm toward the
target layer 42 side and a region that extended from the lower edge
of the target layer 42 by 500 nm toward the diamond substrate
side.
A region near the border between the target layer 42 and the
diamond substrate in the section specimen 55 was observed with a
scanning transmission electron microscope (STEM). From the image
contrast, a region constituted by elements having a specific
gravity smaller than that of the target layer 42 was found near the
target layer 42 but within the diamond substrate 41. This region
was assumed to be a carbon-containing region 45. STEM-EELS line
analysis of the target layer 42 was performed in the thickness
direction and it was confirmed that the assumed carbon-containing
region 45 was distributed by having a thickness of 55 nm to 120
nm.
Next, STEM-EELS evaluation was performed by using an electron loss
energy spectrometer attached to STEM in order to identify the
carbon-containing region featured in the present invention.
As a result, it was found that the normalized sp2 bond
concentration was 96% in the detection region corresponding to the
carbon-containing region and that the normalized sp2 bond
concentration was 1% in the detection region corresponding to the
diamond substrate 41.
A radiation generating tube 102 and a radiation generating
apparatus 101 were produced as in Example 1 but by using the target
9 prepared in Example 3. The radiation generating apparatus 101 was
loaded into the evaluation system 70 for measuring the drive
stability shown in FIG. 7.
The radiation output retention rate of the radiation generating
apparatus 101 of this example was 0.98. This confirmed that the
radiation generating apparatus 101 equipped with the target 9 of
this example does not show significant fluctuations in radiation
output even after long hours of driving history and stable
radiation output intensity is obtained. The radiation generating
apparatus 101 of this example that underwent the test of evaluating
stability of radiation output intensity was disassembled to take
out the target unit 51. No microcrack were observed in the target
layer 42 of the target unit 51.
Example 4
A radiation generating apparatus 101 was produced as in Example 1
except that the steps illustrated in FIGS. 6B-1 to 6B-4 were
performed to prepare a target 9 of Example 4. The radiation output
stability of the radiation generating apparatus 101 was
evaluated.
First, as in Example 1, surfaces of a diamond substrate 41 were
washed as shown in FIG. 6B-1. Then as shown in FIG. 6B-2, a
carbon-containing film 77 composed of diamond-like carbon and
having a thickness of 100 nm was formed by a CVD device on one of
the surfaces of the diamond substrate 41 so as to form a stack.
Next, the stack was placed in an image furnace (not shown) by using
a ceramic holder jig (not shown) composed of alumina. Then a vacuum
atmosphere was created in the interior of the image furnace. The
stack was irradiated with an infrared ray for 10 hours so that the
temperature of the stack was 1400.degree. C. so as to conduct a
heating step. As a result, the carbon-containing film 11 was
transformed into a carbon-containing layer 47 including a sp2 bond.
Next, as shown in FIG. 6B-4, a target layer 42 composed of tungsten
and having a thickness of 6 .mu.m was sputter-deposited on the
carbon-containing layer 47. As a result, a target 9 of Example 4
was obtained.
As in Example 1, a section specimen 55 was cut out from the target
9 of Example 4 by mechanical polishing and FIB processing. The
section specimen 55 was taken to include a region that extended
from the lower edge of the target layer 42 by 300 nm toward the
target layer 42 side and a region that extended from the lower edge
of the target layer 42 by 500 nm toward the diamond substrate side
as shown in FIG. 2B.
A region near the border between the target layer 42 and the
diamond substrate in the section specimen 55 was observed with a
scanning transmission electron microscope (STEM). As a result,
formation of a carbon-containing layer 47 constituted by elements
having a smaller specific gravity than the target layer 42 was
confirmed between the target layer 42 and the diamond substrate 41.
STEM-EELS line analysis of the target layer 42 was performed in the
thickness direction and it was confirmed that the carbon-containing
layer 47 was distributed by having a thickness of 65 nm to 95
nm.
Next, STEM-EELS evaluation was performed by using an electron loss
energy spectrometer attached to STEM in order to identify the
carbon-containing region featured in the present invention.
As a result, it was found that the normalized sp2 bond
concentration was 97% in the detection region corresponding to the
carbon-containing region and that the normalized sp2 bond
concentration was 1% in the detection region corresponding to the
diamond substrate.
A radiation generating tube 102 and a radiation generating
apparatus 101 were produced as in Example 1 but by using the target
9 prepared in Example 4. The radiation generating apparatus 101 was
loaded into the evaluation system 70 for measuring the drive
stability shown in FIG. 7.
The radiation output retention rate of the radiation generating
apparatus 101 of this example was 0.96. This confirmed that the
radiation generating apparatus 101 equipped with the target 9 of
this example does not show significant fluctuations in radiation
output even after long hours of driving history and stable
radiation output intensity is obtained. The radiation generating
apparatus 101 of this example that underwent the test of evaluating
stability of radiation output intensity was disassembled to take
out the target unit 51. No microcrack were observed in the target
layer 42 of the target unit 51.
Example 5
In Example 5, the radiation generating apparatus 101 prepared in
Example 1 was used to produce a radiography system 60 shown in FIG.
3C.
An X-ray image having a high S/N ratio was obtained from the
imaging apparatus 60 of Example 5 since the radiation generating
apparatus 101 in which fluctuation of radiation output was
suppressed was used therein.
It should be noted here that in Examples 1 to 3, identification of
the carbon-containing region and identification of the normalized
sp2 concentration were performed by EELS. However, the
identification technique is not limited to EELS and any other
analysis technique such as Raman spectrometry or X-ray
photoelectron spectrometry capable of separating carbon-carbon
bonds can be employed.
According to the present invention, it becomes possible to provide
a highly reliable transmission type target with which stress
occurring at the interface between a diamond substrate and a target
layer is relaxed and generation of microcracks is suppressed even
when the target is operated at high temperature.
Furthermore, it becomes possible to assuredly provide an anode
potential to the target layer even when operation at high
temperature is conducted and thus a highly reliable radiation
generating tube with which radiation output fluctuations are
suppressed can be obtained. Moreover, a radiation generating
apparatus and a radiography system each having a highly reliable
radiation emission tube can be provided.
While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all such modifications and equivalent structures
and functions.
* * * * *